Thank you, Linda. Great shot of tracks on your friend’s tank. Yes, the terrestrial and freshwater slugs and snails, together with marine limpets and other gastropods including sea slugs (nudibranchs), are related organisms, most of which share the same eating mechanisms. They have rows of small renewable teeth on a ribbon-like radulae (sort of tongue) with which they scrape the algae off the surface. The creatures move their heads from side to side while they scrape the food and move forwards, and this action creates the patterns.